The invention is based on a priority application EP 03 291 500.1 which is hereby incorporated by reference.
The invention is related to an optical amplifier connected to a pump laser with lengths of fibers and isolators between the lengths of fibers to suppress stimulated Brillouin scattering.
The invention is also related to a transmission system using this optical suppression mean.
In addition the invention gives also a rule to optimize the position of isolators in an optical amplifier.
Optical amplifiers of the type in which the amplitude of electric field of light is directly amplified are applicable to the following uses in the optical fiber transmission system and on the optical amplifiers of this type is being made in various areas:    By increasing the output of a light source of the signal light in an optical transmitter, the transmission distance can be increased. When the optical amplifier is used for the light source of local light in an optical receiver on a coherent optical wave communication system, the reception sensitivity can be improved.    By performing optical amplification in the stage immediately before the photoelectric conversion stage, the reception sensitivity can be improved.    By the direct amplification of light, as compared with the method in a conventional optical repeater in which a light signal is once photo-electrically converted into an electric signal and then the electric signal is amplified, it becomes possible to make the repeater itself smaller in size and also to increase the repeater-to-repeater distance.
Optical parametric amplification is carried out by a power transfer from a pump wavelength towards a signal wavelength. This energy exchange depends on phase matching between the waves of the two wavelengths, on their power and on fiber nonlinear coefficient. For ‘small-signal’ e.g. signal with a small power, signal power increases linearly with fiber length. To be efficient the signal power must increase up to the level of the pump power, and than the energy exchange between the wave is reversed. In result the signal wave recharges power back to the pump wave. Then the signal power decreases with length, which makes amplification inefficient.
In order to avoid signal power traveling back to the pump, fibers length in known optical parametric amplifiers is shorter than the length from which signal power decreases. Pump power remains non-depleted during amplification. The efficiency of parametric amplification strongly depends on the frequency shift between signal and pump (through phase matching) wave.
Another problem arise during pumping the parametric amplifier. Parametric amplification requires a high pump power and with the high pump powers nonlinear effects as stimulated Raman and Brillouin scattering occur.
Raman and Brillouin scattering are inelastic processes in which part of the power is lost from an optical wave and absorbed by the transmission medium while the remaining energy is re-emitted as a wave of lower (or higher) frequency. The processes can be thought of as the conversion of an incident photon into a lower energy scattered photon plus a phonon of vibrational energy. Total energy and momentum before and after scattering must be equal, i.e. the incident photon energy is shared between the phonon and the scattered photon. Since the frequency of an optical wave is proportional to its energy, the photon produced by the scattering event has a lower frequency than the incident photon. This frequency downshifted wave is commonly referred to as the Stokes wave. Spontaneous Raman and Brillouin scattering have been observed and measured in bulk samples of material. The growth of the Stokes wave is proportional to the product of the scattering gain coefficient, the intensity of the pump wave and the intensity of any Stokes wave present. In bulk media the Stokes wave quickly disperses as it propagates away from the point of generation. However, single mode optical fibers will support low-loss propagation for waves traveling almost parallel to the fiber axis. Consequently, scattered radiation in either the forward or backward directions relative to the incident wave will be guided within the fiber and will co-propagate with the pump wave over long distances. Under these circumstances, it is possible for the Stokes wave to continue to interact efficiently with the pump wave and exponential growth in the downshifted optical power occurs. For a given length of fiber, gradually increasing the pump power launched into one end will lead to a gradual increase in Stokes power through spontaneous scattering. If the pump power is then increased further, exponential growth in the Stoke power may occur. The input pump power at which the Stokes wave increases rapidly as a function of pump power is termed the stimulated scattering threshold. A major difference between Brillouin scattering and Raman scattering lies in the type of phonon generated—high-energy optical phonons in SRS and lower-energy acoustical phonons in SBS. The difference in frequency between the pump and Stokes waves is therefore much greater in SRS than in SBS. Typical values of the pump-Stokes frequency difference are 10-GHz (˜0.1-nm at 1550-nm) for SBS and 13-THz (˜110-nm at 1550-nm) for SRS. Another key distinction between the two effects is that the scattered wave due to SBS travels predominantly backwards. The SBS Stokes wave emerges from the input end of the fiber whereas the Stokes wave due to SRS travels forwards with the pump wave. Both SBS and SRS have threshold pump powers above which power transfer to the Stokes wave increases rapidly. In SBS this means that the amount of optical power leaving the far end of the fiber no longer increases linearly with the input power. The maximum launch power becomes clamped and excess power is simply reflected back out of the fiber. For long distance and highly branched fiber links, it is important that as much power as possible can be launched into the fiber to compensate for attenuation and power splitting. Limits on the maximum launch power due to SBS must therefore be avoided. The same problems arise when a parametric amplification scheme is used.
One way to avoid pumping above the SBS threshold is to broaden the pump spectrum. This is normally done by a phase modulation of the pumping signal. This need some additional effort to apply the special modulation formats on the pump and this only to broaden the spectrum, as explained in Y. Aoki, K. Tajima, I. Mito, “Input Power Limits of Single-Mode Optical, Fibers due to Stimulated Brillouin Scattering in optical communication, systems”, J. Lightwave technology, vol.6 no.5, p710. It is the objective of the invention to overcome the problems with high pump signals arising SBS in an optical amplifier.